Hand-held radiotelephone transceivers are currently available for operation in either the 900 MHZ or 1.8 gHz bands. Since the size and weight of a hand-held unit must be kept to a minimum, it is desirable to employ as small and low voltage a battery as possible, typically 2.7 volts, and to employ circuits which reduce the battery current drain in order to maximize useful operating time. Such transceivers, however, also use a form of vector or phase modulation which requires that the RF amplifier stage preceding the final power amplifier be operated in class A linear mode to prevent phase shift error. Such a stage typically employs an emitter-follower at its output which must supply 1 milliwatt drive power into the low impedance (usually 50 ohms) presented by the single-ended filter leading to the final power amplifier. To maintain the RF amplifier stage in its linear, class A region, where the peak-to-peak excursion of the driving voltage cannot be permitted to clip the signal, it has heretofore been necessary to make sure that the transistors of the emitter-follower output neither saturate nor cut off. Accomplishing these goals has heretofore required that these transistors draw a large dc standby current equal, theoretically, to at least half the peak-to-peak output current. In practical applications the value of standby current is much larger.
Also, because of the use of a low voltage battery, the RF amplifier stage is in the amount of voltage swing that can be delivered to the base of the emitter-follower output transistor, typically not more than about 300 millivolts peak-to-peak. Since the load to be driven is single-ended, a coupling arrangement such as an air core RF transformer or a phase shifting circuit is required to convert the differential output to a single ended output. Unfortunately, both types of coupling arrangements tend to operate best in only one frequency band. It is thus very difficult to have the same RF amplifier circuit work at both the 900 MHZ and 1.8 or 1.9 gHz bands.
It would be extremely advantageous to provide an RF amplifier capable of delivering the required output power to a single ended load without requiring the use of transformers, without requiring the use of a large standby current to maintain amplifier linearity and which operates efficiently at more than one RF band so that a hand-held radiotelephone receiver could in fact be operated at more than one, such as both the 900 MHZ and 1.8 or 1.9 gHz bands.